Abrupt disease onset and severe metabolic disorders are main characteristics of fulminant type 1 diabetes. Diffusion-weighted magnetic resonance imaging (DWI) is an imaging technique that reflects restricted diffusion in organs and can detect mononuclear cell infiltration into the pancreas at the onset of the disease. Fourteen patients with fulminant type 1 diabetes who underwent abdominal magnetic resonance imaging were recruited for the measurement of apparent diffusion coefficient (ADC) values of the pancreas that were compared with those of 21 non-diabetic controls. The ADC values of all parts of the pancreas were significantly lower in fulminant type 1 diabetes than in controls (head, 1.424 ± 0.382 × 10-3 vs. 1.675 ± 0.227 × 10-3 mm2/s; body, 1.399 ± 0.317 × 10-3 vs. 1.667 ± 0.170 × 10-3 mm2/s; tail, 1.336 ± 0.247 × 10-3 vs. 1.561 ± 0.191 × 10-3 mm2/s; mean, 1.386 ± 0.309 × 10-3 vs. 1.634 ± 0.175 × 10-3 mm2/s) (p < 0.01). The best cut-off value indicated that the sensitivity was 86% and the specificity was 71% when using DWI, which was also efficient in two atypical patients with fulminant type 1 diabetes without elevated levels of exocrine pancreatic enzymes or with high HbA1c levels due to the preexistence of type 2 diabetes. The ADC values were significantly correlated to plasma glucose levels and arterial pH, and tended to increase with the lapse of time. DWI may be an additional tool for making an efficient diagnosis of fulminant type 1 diabetes.